The beta 1-adrenoceptor antagonist, betaxolol, is not released from the heart of the anaesthetized dog during sympathetic nerve stimulation.

1. We investigated the hypothesis that the beta 1-adrenoceptor antagonist, betaxolol, can be accumulated by cardiac sympathetic nerve endings and then released together with noradrenaline during accelerans nerve stimulation. 2. Dogs were chronically treated with betaxolol (1 mg kg-1 daily, s.c.) for 7 days. Twenty four hours after the last dose, there was a significant retention of betaxolol in the heart of these dogs treated chronically with the beta 1-adrenoceptor antagonist. However, during in vivo accelerans nerve stimulation, the concentration of betaxolol in the coronary sinus was not modified, whereas the noradrenaline concentration increased significantly. 3. Chronic betaxolol treatment antagonized the tachycardia induced by electrical stimulation of the cardiac accelerator nerves or by intravenous isoprenaline. However, the tachycardia induced by nerve stimulation was not antagonized to a greater extent than that induced by isoprenaline. 4. These findings are discussed in relation to a similar in vivo study in dogs treated with propranolol, in which the drug was found to be released into the coronary circulation during stimulation of the accelerans nerve.